The invention relates to a process for oxidizing methanol or methanol-containing waste streams. More particularly, this invention is directed to a method for oxidizing methanol in a gas stream (for example, pure methanol or a paper pulp mill waste stream of methanol, methyl mercaptan and other waste products) to form formaldehyde (CH2O) by passing the gas stream through a catalyst bed containing, in a specified distribution, at least two metal oxide supported catalysts.
A method for converting methanol in a methanol gas stream or in a waste gas stream to formaldehyde involves contacting the gas stream with any one of a variety of supported metal oxide catalysts under oxidizing conditions. The constituents of the gas stream can be fully or partially oxidized. However, partial oxidation of methanol to formaldehyde is preferred.
Vanadia-titania (V2O5 supported on TiO2) and molybdena-titania (MoO3 supported on TiO2) supported catalysts are two catalysts developed for selectively oxidizing methanol to formaldehyde. Both of these catalysts are highly active. The vanadia-titania catalyst is the more active of the two.
Unfortunately, both of these catalysts have disadvantages associated with their use. For example, vadadia-titania catalysts exhibit a high catalytic activity. Accordingly, such catalysts oxidize methanol to formaldehyde and continue to further oxidize the formaldehyde into undesirable oxidation products including carbon monoxide (especially when a high concentration of formaldehyde is available). Consequently, formaldehyde yield is undesirably lowered.
One of the disadvantages associated with molybdena-titania catalysts is due to the volatility of molybdenum trioxide (or other volatile Mo species) contained therein. In particular, the oxidation of methanol to formaldehyde is highly exothermic. The exothermic nature of the oxidation reaction in combination with the high catalytic activity of the molybdena-titania catalyst (and/or the high catalytic activity of other catalysts used in combination with molybdena-titania catalysts) creates hot spots on the catalyst surface during methanol oxidation to formaldehyde. As more methanol is oxidized, the temperature at these hot spots continues to increase. Finally, these hot spots reach a temperature sufficient to sublime molybdenum trioxide (or other volatile Mo species present in the catalyst).
When molybdena-titania catalysts are distributed in an upstream region of a catalyst bed, molybdenum trioxide (or other volatile Mo species) tends to migrate to the cooler downstream regions of the catalyst bed where it condenses into crystalline structures, such as needles. Ultimately, accumulation of these crystalline structures in the downstream regions of the catalyst bed impede the flow of the incoming feed stream (containing methanol) being introduced into the catalyst bed. Thereby, methanol oxidation and catalytic activity are undesirably suppressed.
For MoO3 catalysts having less than monolayer coverage on TiO2, the aforementioned Mo/MoO3 volatility and sublimation may not be problematic. However, for MoO3 catalysts with greater than monolayer coverage on TiO2 or the like, Mo/MoO3 volatility and sublimation is typically problematic. Nevertheless, MoO3/TiO2 catalysts offer some advantages (e.g., over vanadia-titania catalysts) including their lower activities for oxidizing formaldehyde. Accordingly, formation of further oxidation products of formaldehyde (e.g., CO, CO2, and the like) may be avoided or attenuated by using MoO3/TiO2 catalysts rather than, for example, using V2O5/TiO2 catalysts.
Even with their associated disadvantages, there have been numerous attempts to use these catalysts (e.g., vanadia-titania and molybdena-titania sometimes in combination with other catalysts) to achieve a high conversion of methanol together with a high selectivity for formaldehyde. Freidrich et al., U.S. Pat. No. 3,978,136, discloses a combination of MoO3 and ferric oxide (Fe2O3) on a sole titania support where the molar ratio of Mo:Fe is less than 10:1. This catalyst exhibits conversions of approximately 99% with a selectivity of approximately 92% at a reactor temperature of approximately 290xc2x0 C. However, this catalyst system suffers from the aforementioned molybdenum trioxide volatility problem. Hoene, U.S. Pat. No. 4,343,954, discloses a sequential two catalyst, two reactor system using a silver catalyst (in a first reactor) and a metal oxide catalyst selected from a group including vanadium and molybdenum (in a second reactor). However, the use of the silver catalyst requires an extremely high temperature (approximately 600xc2x0 C.) and a subsequent cooling step between the two reactors. As such, silver catalyst systems are prohibitively expensive to use. Windawi U.S. Pat. No. 4,421,938, discloses a combination of MoO3 and any of a list of other metal oxides (not including vanadium) on the same alumina/inorganic oxide support. Sarup et al., U.S. Pat. No. 5,118,868 and U.S. Pat. No. 5,217,936, discloses a catalyst comprised of mixed oxides of molybdenum and another metal oxide (M), including vanadium, with a molar ratio of Mo:M from 1:1-5:1, where the mixed oxides are supported on corrugated sheets of fibrous material. However, none of these approaches adequately solve or address the volatility problem associated with the use of MoO3 or the undesirable oxidation of formaldehyde to its further oxidation products associated with the use of V2O5.
It is therefore an object of the present invention to provide a method for the partial oxidation of methanol or methanol containing waste gas streams to formaldehyde by passing the gas stream and an oxidizing agent, at oxidizing conditions, over a catalyst bed of two or more catalysts distributed in a manner which substantially alleviates and/or eliminates the aforementioned undesirable further oxidation of formaldehyde (e.g., carbon monoxide formation or other formaldehyde oxidation products) and molybdenum trioxide volatility problems (e.g., the formation of crystalline needles in the downstream region).
It has surprisingly been discovered, for example, that the use of vanadia-titania catalysts positioned upstream in the catalyst bed of a fixed bed reactor (upstream regions being the regions closer to the reactor""s entrance) combined with molybdena-titania catalysts positioned further downstream in the reactor""s catalyst bed yields a methanol conversion greater than 99% while substantially eliminating the undesirable further oxidation of formaldehyde to carbon monoxide (or other formaldehyde oxidation products) together with substantially avoiding the aforementioned MoO3 volatility problems.